In recent years, an MR (magnetoresistive) element has been used as an element for the head of a magnetic recording medium such as a hard disk drive (HDD) and a floppy disk drive FDD). The head that uses an MR element has a larger output for reproduction as compared to a head that uses a conventional thin-film element, therefore, disk recording density of the magnetic recording medium can largely improve.
Herein the MR element according to the present invention means an element that shows a magnetoresistive (MR) effect in which resistance of the element varies when an external magnetic field is applied to the element. Examples of such elements are a GMR (giant magnetoresistive) element or a TMR (tunneling magnetoresistive) element.
FIG. 9 shows configuration of a signal amplifying circuit for an MR element based on the conventional technology.
In FIG. 9, designated at the reference numeral 1a is a first MR element, at 1b a second MR element, at 2a and 2a' a pair of first switching circuits, at 2b and 2b' another pair of first switching circuits, at 2c and 2c' a pair of second switching circuits, at 3 a constant current source, at 4 an amplifier, at V1 a first potential (a power-supply potential in FIG. 9), at V2 a second potential (a ground potential in FIG. 9), at MR1 a first terminal of an MR element, at MR2 a second terminal of the MR element, at OUT1 a first output terminal of the amplifier 4, and at OUT2 a second output terminal of the amplifier 4, respectively.
Next, operation of the signal amplifying circuit for an MR element shown in FIG. 9 is described.
The first switching circuits 2a, 2a' and the other first switching circuits 2b, 2b' are paired, respectively. Those first switching circuits operate in such a way that, when one of the paired switching circuits (e.g., 2a) is ON the other (e.g., 2a') is also ON, and when one of the paired switching circuits is OFF the other is also OFF. Further, those switching circuits are controlled by a control circuit not shown in the figure such that when the first switching circuits 2a, 2a' are ON the other first switching circuits 2b, 2b' are OFF, and when the first switching circuits 2a, 2a' are OFF the other first switching circuits 2b, 2b' are ON. Through the controls provided as described above, a current is supplied to either the first MR element 1a or the second MR element 1b.
Furthermore, the second switching circuits 2c, 2c' are paired, and controls are provided by the control circuit not shown in the figure such that when the first switching circuits 2a, 2a' are ON the second switching circuits 2c, 2c' are connected to respective terminals a, and when the other first switching circuits 2b, 2b' are ON the second switching circuits 2c, 2c' are connected to respective terminals b.
Next, description is made the operation when the first MR element 1a is selected (when the first switching circuits 2a, 2a' are ON). In this case, as described above, the other first switching circuits 2b, 2b' are OFF, and further the second switching circuits 2c, 2c' are connected to respective terminals a. Therefore, a current is supplied to the first MR element 1a from a power supply unit as the first potential V1 through the first terminal MR1 of the MR element by the constant current source 3. The first terminal MR1 and the second terminal MR2 of the first MR element 1a are connected to respective inputs of the amplifier 4 and, a signal from the first MR element 1a is amplified by the amplifier 4 and outputted from the first output terminal OUT1 as well as the second output terminal OUT2 of the amplifier 4.
FIG. 10 shows a configuration using emitter-follower transistor circuits for the components corresponding to the pair of second switching circuits 2c, 2c' in the conventional signal amplifying circuit for an MR element in FIG. 9. In FIG. 10, the reference numerals are assigned to the same sections corresponding to those in FIG. 9.
In FIG. 10, the first terminal MR1 of the first MR element 1a is connected to the base of a NPN transistor Tr1, and the second terminal MR2 of the MR element is connected to the base of a NPN transistor Tr2, and the emitter of the NPN transistor Tr1 and the emitter of the NPN transistor Tr2 are connected to respective inputs of the amplifier 4.
The conventional of signal amplifying circuit for an MR element in FIG. 10 performs the same operation as that of the circuit in FIG. 9, but comprises the emitter-follower transistor circuit for the components corresponding to the second switching circuits 2c, 2c', thus the circuit operates such that signals of an MR element selected by the first switching circuits 2a, 2a' or by the other first switching circuits 2b, 2b' are inputted into the amplifier 4 as they are.
However, the conventional signal amplifying circuit for an MR element described above has drawbacks. Namely, in the conventional signal amplifying circuit for an MR element, signals comprising DC and AC current components obtained from both the terminals of the MR element are directly inputted into the amplifier, so that, when an offset voltage (voltage difference) arises in the signals from the MR element, an imbalance arises at the two inputs of the amplifier. Therefore, the dynamic range becomes narrow or amplification of a weak signal cannot accurately be performed.